List Stream 的常规用法

1. 常规元素去重

// 遍历后判断赋给另一个List集合，保持原来顺序 public static void ridRepeat1(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(); for (String str : list) { if (!listNew.contains(str)) { listNew.add(str); } } System.out.println("listNew = [" + listNew + "]"); }

// Set集合去重，保持原来顺序 public static void ridRepeat2(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(); Set set = new HashSet(); for (String str : list) { if (set.add(str)) { listNew.add(str); } } System.out.println("listNew = [" + listNew + "]"); }

// Set去重 由于Set(HashSet)的无序性，不会保持原来顺序 public static void ridRepeat3(List<String> list) { System.out.println("list = [" + list + "]"); Set set = new HashSet(); List<String> listNew = new ArrayList<String>(); set.addAll(list); listNew.addAll(set); System.out.println("listNew = [" + listNew + "]"); }

// Set通过HashSet去重（将ridRepeat3方法缩减为一行） 无序 public static void ridRepeat4(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(new HashSet(list)); System.out.println("listNew = [" + listNew + "]"); }

// Set通过TreeSet去重 会按字典顺序重排序 public static void ridRepeat5(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(new TreeSet<String>(list)); System.out.println("listNew = [" + listNew + "]"); }

// Set通过LinkedHashSet去重 保持原来顺序 public static void ridRepeat6(List<String> list) { System.out.println("list = [" + list + "]"); List<String> listNew = new ArrayList<String>(new LinkedHashSet<String>(list)); System.out.println("listNew = [" + listNew + "]"); }

使用ＪＤＫ1.8的去重

 //利用java8的stream去重
  List uniqueList = list.stream().distinct().collect(Collectors.toList());
  System.out.println(uniqueList.toString());

2. 对象去重

解决对象去重，可以利用for循环遍历的方式进行判断去重，但今天我不准备探究这种方法，要使用的是如下两种：

//根据name属性去重 List<User> unique1 = userList.stream().collect( collectingAndThen( toCollection(() -> new TreeSet<>(comparing(User::getName))), ArrayList::new));

System.out.println(unique1.toString());

//根据name,age属性去重 List<User> unique2 = userList.stream().collect( collectingAndThen( toCollection(() -> new TreeSet<>(comparing(o -> o.getName() + ";" + o.getAge()))), ArrayList::new) );

System.out.println(unique2.toString());

2.2 对象中重写equals()方法和hashCode()方法

//重写equals方法 @Override public boolean equals(Object obj) { User user = (User) obj; return name.equals(user.getName()) && (age==user.getAge()); }

//重写hashCode方法 @Override public int hashCode() { String str = name + age; return str.hashCode(); }

3. equals()方法和hashCode()方法探究

/** * Compares this string to the specified object. The result is {@code * true} if and only if the argument is not {@code null} and is a {@code * String} object that represents the same sequence of characters as this * object. * * @param anObject * The object to compare this {@code String} against * * @return {@code true} if the given object represents a {@code String} * equivalent to this string, {@code false} otherwise * * @see #compareTo(String) * @see #equalsIgnoreCase(String) */ public boolean equals(Object anObject) { if (this == anObject) { return true; } if (anObject instanceof String) { String anotherString = (String)anObject; int n = value.length; if (n == anotherString.value.length) { char v1[] = value; char v2[] = anotherString.value; int i = 0; while (n-- != 0) { if (v1[i] != v2[i]) return false; i++; } return true; } } return false; }